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Milling Cutters.                           *75

the use of the expansible cutter in Fig. 183, which is divided, at
ab by a plane slightly inclined to that of the cutter, and has thin
discs inserted to preserve the normal width. If a & were at right
angles to the axis a strip of uncut material would be left on the
work, which is here obviated, besides which, various widths of
grooves may be cut. Further, if required, two mills may be
placed on one spindle, the teeth being interlocked, and a groove
of about twice the former width thereby cut, but it is important
that the mills "be of exactly the same diameter, obtained by
grinding them together on the same spindle. Fig. 184 shews a
pair of heading or twin mills for forming the sides of hexagon nuts
or other parallel work, the width being varied by the insertion of
suitable packing, In Fig. 185, A is a mill for grooving a screw
tap, B for fluting a rimer, and c an angular mill for cutting the
teeth of other mills. (See Appendix //".,,/. 8r i.)

When a grooving mill is allowed to cut on its side only, sayr
when fixed in a vertical machine, it is termed a face cutter, but
such an application is not desirable.

The steel or * blank ' to form the cutter is turned to correct
diameter while soft, and the teeth then cut. It is next tempered*
to a straw colour, and the edges are finished by grinding with a
small emery wheel of the same shape as the mill c, Fig. 185.
Great care must be taken to avoid cracking while hardening", but
distortion is now removed by grinding the hardened mill.

Fig. 186 represents a cutter for forming the teeth of spur
wheels by removing the interspaces, a is the relief angle or bottom
rake, a side rake being provided by cutting the profile in an arc
eccentric to that of the point path when rotating. Thus b is th e
centre for formation of the cutting tooth surface, while c is the
centre of rotation. Now d d and e e are curves struck from fr, aad
sections on each of these lines would be rectangular, but a
section on de must take the shape shown at./ because dħ </* Is-
greater than el eħ as seen in the end view. But as d e is the path
of the point d during the revolution of the cutter, clearance or
relief angle is therefore given at the side, and the cutter is said to-
be ' backedoff/ Of course this method can. only be used with
cutters of tapering profile; it enables us to preserve both form
aad width of cutting tool, however much is removed from the
face, and is art improrernent on the old cutter, which became